It is common to use abrasive blast cheaning, i.e., "shot-blasting", to clean and prepare large areas of ferrous articles, especially hot rolled steel, as a preparation for painting. It is becoming increasingly common, the health and safety and environmental reasons, to inject water into the air/abrasive stream, thus suppressing dust and fumes which are otherwise dangerous and a nuisance to the operator and the surroundings. The resultant process may be termed "wet blasting". An advantage of wet blasting is that it appears to remove soluble corrosive salts that are not removed by dry blasting. However wet blasting suffers from a major disadvantage which is that the freshly blasted steel surface is very clean and reactive but is damp and so rusts freely. The resultant rust bloom on the surface is regarded as detrimental to the properties of subsequently applied paint coatings.
It is known to include an inhibitor in the injected water with the intention of preventing this rusting.
The inhibitors that have been proposed commercially are sodium nitrite and a polyphosphate such as material sold under the trade name Nalfloc 918. These are both very sensitive to concentration problems. Thus sodium nitrite can reduce the rate of rust formation if applied at the right concentration but if it is applied at a concentration that is too low or too high it can accelerate rust formation. Polyphosphate permits rust formation if it is applied at a concentration that is too low and if it is applied at a concentration that is too high, or if the wash water accumulates and dries on the surface, the adhesion of subsequently applied paint is unsatisfactory. The films are water soluble and are liable to be rinsed off the surface.
It is well known to form phosphate coatings on ferrous surfaces by application to the surface of an acidic phosphate solution optionally containing heavy metal ions. In order to obtain a protective coating in a shorter time period, it is necessary to include nitrate or other accelerator system. However such solutions are not satisfactory for inclusion in the blast water during wet blasting as either they do not prevent rusting or they form a coating which does not provide satisfactory adhesion of subsequently applied paint.
It would be desirable to be able to prevent rusting during wet blasting and to promote good adhesion of subsequently applied paint, and in particular it would be desirable to provide such a system wherein satisfactory results can be achieved over a range of concentrations and irrespective of whether or not the coated surface is rinsed with water after the treatment.